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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 62 



To verify this law of resistance, it was decided to test in the wind 

 tunnel a series of thin disks placed normal to the wind. Disks of 

 sheet brass 1/16 inch thick with square edges and diameter 2, 3, 4, 5, 

 and 6 inches were used. The resistance of each disk was measured 

 for a series of speeds. The measurements were repeated with the 

 face of each disk reversed and the results averaged. 



If resistance be correctly represented by the formula above, the 

 constant K computed from the observed resistances should remain 

 constant. In figure 36 the values of K computed from each observa- 

 tion are plotted on the product VD as abscissae, when D is the diam- 

 eter in inches. It appears that K does not remain constant even for 

 a given disk. The points represent actual observations, and it is seen 



io 30 tic SO 60 7e 80 90 IOC no ize 130 IHo ISO I60 110 180 190 



Fig. ^j.- — ^Resistance coefficient as a function of VD. 

 R 



K: 



where 



pAV 



R = force in pounds. 



P = density in pounds per cubic foot. 



A = area in square inches. 



V = velocity in miles per hour. 



D = diameter in inches. 



that below a value of VD of 40, K becomes very erratic. A great 

 many check observations were taken in this region, but the flow seems 

 to be unstable and K cannot be determined with precision. The mean 

 curves are replotted on figure 37 on VD as abscissas, and again on 

 figure 38 on V as abscissae. The critical velocity apparently dis- 

 covered seems to be about 9 miles per hour for all the plates. 



Theoretically, if the resistance due to viscosity be important the 

 coefficient K should be a function of the product VD. Thus, Lord 

 Rayleigh has suggested' that where both density and viscosity are 



^ Phil. Mag., p. 66, July, 1904. 



